iSiLucia: Back channel and weir design

iSiLucia: Back channel and weir design

Van Alphen, S.
Reijmerink, B.
Stoeten, K.
Tuin, H.
Vinke, D.J.

Civil Engineering and Geosciences

Hydraulic Engineering

2011-10-01

This report describes the process of the design of the “Waterdraer”, a new connection between the Mfolozi and the St Lucia estuary. The different process steps of Systems Engineering have been used as a guideline during the design. In short, the main goal of the Waterdraer is to transport fresh water into St Lucia without a long extensive back up of water in the Mfolozi. In the beginning of the report the following research question was defined: · What is the most optimal solution for restoring and enhancing some of the features of the original ecologic St Lucia estuary system, in relation to the short term possibilities? The St Lucia estuary and surroundings are part of the GEF project in the iSimangaliso Wetland Park. The GEF project involves many different stakeholders and complex problems. Even in the short term there are a lot of interests at stake. First, there is the need for fresh water into the St Lucia estuary, the Narrows, and lakes. Secondly, there are the sugarcane farmlands that should be “protected” against flooding. Thirdly, there is a need for biologic and ecologic exchange between the sea and iSimangaliso Wetland Park. Finally there is a need for an open St Lucia mouth connection for local fishery. The difficulty of these problems is that they are all interdependent. However, the four problems are also conflicting with one another. The current situation shows that the old Back Channel is not sufficient enough to provide enough water inflow without backing up water in the Mfolozi floodplain. Even when the Mfolozi berm height is sustained the discharge of the “natural” weir in the old Back Channel is too low. As a result the water levels could rise too fast and would lead to an artificially or even a natural breach. Even a small flood could not be sustained to prevent a (artificially) breach of the berm. An open Mfolozi berm results in the malfunctioning of the old Back Channel. In the past years a lot of knowledge about the problems and systems has been gained. Mismanagement during the past 50 years becomes gradually accepted and the iSimangaliso Wetland Park authority has changed the management strategy of the iSimangaliso Wetland Park. One of the preferred long term options is bringing back the combined mouth dynamics. Currently, this is not an option because management conflicts and uncertainties about such impact are unsolved. The improved Back Channel (the Waterdraer) anticipates to the customer needs by providing an easy implementable short term solution. The Waterdraer its main goal is to bring enough fresh and sediment free water into St Lucia to counteract the hypersalinity during droughts. In addition, it should limit the effects of flooding on the lower floodplains. The Waterdraer variant consists of the widening of the recently excavated new Back Channel. The bottom level is decreased towards 0m GMSL, which is assumed to be the current height of the “main” channel of the Back Channel. The threshold of (artificially) breaching the Mfolozi berm is set at +1.8m above GMSL. The proposed threshold level is depended on two factors. Firstly the amount of flooding allowed on the floodplains and secondly the largest flood wave that has to be retained. A new weir inside the new Back Channel is designed as the control notch of the system. The weir creates a “controllable” situation and minimizes the effects of sediment transport. The crest height and width of the overflow determine the size of the flow into St Lucia. The width of the weir is based on the maximum flow velocity at which erosion occurs in mangrove forests, and on the minimum flow required to fulfill the requirements. A stilling basin of soft materials (logs) is designed to dissipate the energy of the water that flows over the weir. Modelling in SOBEK shows that the improved Back Channel could have provided a flow of approximately 64Mm3 considering the discharge of the last 4. months. The latter is based on the condition that the maximum water level behind the Mfolozi berm would be increased to +1.8m GMSL. An interesting remark is that the water level has not exceeded +1.5m GMSL during this period. In addition, the artificial breach of the Mfolozi berm in July 2011 has been kept out the model in order to simulate the natural situation of the Mfolozi mouth. Lawrie and Stretch (2011) mention that hypersaline conditions in the St Lucia lakes can be prevented if the Back Channel provides a water inflow of at least 5Mm3/month, which is 60Mm3/year. The SOBEK model and a simple water balance model suggest that even small floods (single peak flood waves with a peak height . 75m3/s) are retained behind the berm. Finally, the Waterdraer could drop water levels in the Mfolozi from +1.8m GMSL back to + 1.5m GMSL in five days if such a flood occurs. Floods above 75m3/s result in a natural breach which means long flooding periods on the floodplains are no longer present. In short, the Waterdraer offers an easy implementable short term solution that counteracts hypersalinity in the St Lucia lakes. Furthermore, it minimizes the effects of flooding on the floodplains that lie above +1.5m GMSL. Due to the retention basin behind the Mfolozi berm sediment inflow into the Narrows is reduced to acceptable levels. The effect on marine distribution is considered to be minimal. In comparison with a combined mouth the new Back Channel does not provide enough inflow to breach the St Lucia mouth by itself.

St. Lucia
salt intrusion
hypersalinity
iSimangaliso
Mfolozi

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TU Delft, Section Hydraulic Engineering

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© 2011 The Author(s)